One of the largest error sources in all radio frequency (RF) positioning systems is multipath. Multipath refers to the phenomenon of a signal reaching a receive antenna via two or more paths. Typically, a receive antenna receives the direct signal and one or more signals reflected from structures in the receive antenna's vicinity. The subsequent range measurements determined by a position receiver are the sum of the received signals, which are generally measured “long” due to the delayed nature of the multipath reflections. Therefore, multipath reflections cause code-based pseudorange biases in location networks which can substantially degrade absolute position accuracy measured by a position receiver. Furthermore, multipath reflections which arrive at the receive antenna with phases different to those of the direct signal will sum destructively with the direct signal, and therefore cause a loss of received signal power, known as signal fading. Moderate signal fading causes measured carrier phase errors of up to +/−90 degrees, and pseudorange biases in the tens of metres. Severe signal fading causes receiver tracking loop destabilization, cycle slips, pseudorange biases in the hundreds of metres, and possible complete loss of lock on the positioning signal. Moreover, the unintentional measurement of off-axis multipath signals corrupts receiver Doppler measurements, leading to significant degradation in the accuracy of velocity and carrier range measurements in a position receiver. This makes velocity measurements read “low”, and integrated carrier phase measurements range “short”.
Received signal-to-noise ratios of positioning signals also affect the measured precision of ranging signals. In general, the greater the received signal strength the better accuracy of the measurement. Signal-to-noise ratios are degraded by (1) increasing distance from the transmission source, (2) signal attenuation caused by line-of-sight obstructions, such as buildings and foliage, (3) multipath signal fading, and (4) an increased noise floor caused by intentional or unintentional signal jammers emitting signals on the positioning signal frequency.
Prior art methodologies for noise and multipath mitigation using antenna design have focused on two key areas; (1) multipath limiting antennas, and (2) Programmable Multi-beam Antenna Arrays. Multipath limiting antennas shape the receive antenna gain pattern to reduce the strength of reflected off-axis signals. The most common form of this antenna being the so-called choke ring antenna used in GPS applications for mitigating satellite signal ground reflections. Multipath limiting antennas traditionally position a directional gain antenna in a fixed orientation, generally positioned facing away from the offending reflective surface (the ground in the case of the choke ring OPS antenna). This method has limited application in high multipath environments, such as indoors or urban areas, where signals reflect from many directions including buildings, walls, floors, ceilings, furniture, and people.
Programmable Multi-beam Antenna Arrays dynamically shape the receive antenna gain pattern to reduce the effect of interference sources, such as intentional signal jammers, and also reduce the affect of multipath signals. Programmable Multi-beam Antenna Arrays either; (1) combine a plurality of antenna elements to form a gain null in a single antenna gain pattern, or (2) combine a plurality of directional gain antennas, each focused on one of the GPS satellites, to form a plurality of peaks in a single antenna gain pattern, or (3) individually monitor a plurality of directional gain antennas, each focused on one of the GPS satellites, through a matrix of receiver circuitry. A Programmable Multi-Beam Antenna Array, which produces a dynamically adjustable gain null in its antenna gain pattern, has application for mitigating the effect of signal jamming and thus improving received signal-to-noise ratios by decreasing antenna gain in the direction of the noise source. However, this antenna array has limited application for multipath mitigation in high multipath environments, where multipath signals reflect from many directions. A Programmable Multi-Beam Antenna Array, which produces a plurality of dynamically adjustable gain peaks in its antenna gain pattern, has application for mitigating the effect of signal jamming and improving received signal-to-noise ratios by increasing gain in the direction of the satellites and decreasing gain in the direction of the noise source. However, this antenna array has limited application for multipath mitigation in high multipath environments, where a significant amount of multipath is received through off-axis antenna gain peaks, which are intended for the reception of other satellite positioning signals. Individually monitoring a plurality of directional gain antennas through a matrix of receiver circuitry has application for mitigating the effect of signal jamming and improving received signal-to-noise ratios, and also mitigating the affect of multipath. However, a matrix of receiver circuitry has many disadvantages, including: (a) the potential for time-variant group delay and line biases being introduced into individual positioning signal measurements due to the use of disparate receive paths. These delays change with variations of component temperature and supply voltage, thus causing time variant ranging errors and subsequent position inaccuracies in the position receiver Position Velocity Time (PVT) solutions; (b) heavy power consumption due to the additional radio frequency (RF) circuitry, making the position receiver unsuitable for applications where battery weight and size are restricted; (c) the requirement for proportionally more electronic components than a standard single front-end receiver design, making the position receiver relatively expensive to produce; and (d) the large form factor required to house the additional receive circuitry, making the receiver larger than a standard single front-end receiver. A system that can provide positioning signals free from the encumbrance of severe multipath and degraded signal-to-noise ratios, without any of these constraints, is highly desirable. The present invention achieves this desirable goal by spatially synchronizing a Time Division Multiple Access (TDMA) Adaptive Directional
Antenna Array to a chronologically synchronous Time Division Multiple Access (TDMA) location network, as described below.